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A. Insulin is a hormone manufactured in the pancreas that helps the body turn sugar into energy. In type 1 diabetes, the body mistakenly destroys the cells that manufacture insulin, and in type 2 diabetes, the body either stops making enough insulin or can no longer use it properly. In either case, the result is chronically high blood sugar, which can harm the eyes, kidney, nerves, and heart.

George King, chief scientific officer at the Joslin Diabetes Center, says that doctors usually diagnose and manage diabetes by testing blood sugar alone, but tests that measure insulin levels are sometimes used to determine which type of diabetes a person has, or whether a person has other insulin-related conditions such as low blood sugar. Measuring insulin production is also important in diabetes research — for instance, to see if treatments for type 1 diabetes can restore the insulin-producing cells in the pancreas.

Insulin can be measured directly with a blood test using an antibody that binds to insulin in the blood. But King explains that people with type 1 diabetes, and anyone who takes artificial insulin, produce their own antibodies to insulin that can interfere with the test results. Because of this, a different test is often preferred that measures C-peptide, a byproduct of insulin production that serves as a marker for how much insulin the body makes. A C-peptide test is sometimes required by insurers for patients applying for an insulin pump, a medical device that automatically delivers insulin.